As telephone system technology is evolving toward a future high speed Integrated Services Digital Network (ISDN) for providing complete wideband interconnectivity, a current need remains for systems which can provide high-speed data links during the potentially long delay period before ISDN implementation. This need has been confronted in various ways by the AT&T "ACCUNET" Switched 56 Service, the AT&T "Dataphone" Digital Service, the prospective AT&T Circuit Switched Digital Capability (CSDC) and the "Datapath" system, offered by Northern Telecom, Inc. of Research Triangle Park, N.C. The last is an all digital data connection system based on digital circuit switches, special data units and line cards to enable data from digital sources to be transmitted at high rates via public digital telephone switches. It has been placed in widespread use because it not only enables high-speed transfers between modern data processing equipment but also provides advanced circuit-switched data services not only for central office switches but also for private branch exchanges (PBX's). It can also interface with the CSDC system with certain constraints. Being an all digital system, voice inputs have not been incorporated, even with pulse code modulation. This limitation is not acceptable for many present users and applications, since voice messages and signalling equipment are in such widespread current use. The ability to employ both types of messages means that the customer base could be significantly expanded and that service charges for high speed digital service could be drastically reduced.
The "Datapath" system is accessed by a combination of data units on the user's premises which interface to terminals on public and private networks, standard non-loaded cable pairs, and line cards at the channel banks. A wide variety of user equipment, with different data rates and interfaces, can be supported with these components. Despite disparities in data rates a common data transmission rate of 56 KBP/s (which plus clocking pulses gives a full rate of 64 KBP/s) is provided, with error correction data and synchronizing pulses being inserted where bandwidth is available. These functions are handled under a "T-link" protocol which also uses sequences of interchanged queries and responses to identify an acceptable frequency for both sender and receiver. Although dialing of a called number can be by keyboard at the terminal or data unit, or via a modem protocol, once the connection is made transmissions are entirely encoded and duplex in nature. Voice messages on this system would return back an echo from the called party's end that is unacceptable in modern systems, because of mismatches and feedback at the hybrid circuit and telephone handset.
The objective of communicating both voice and data signals by a common network was initially met by using low speed modems to express digital data in analog form for analog switching systems. The troublesome problem of voice echo was long ago overcome in analog networks by introducing echo suppression circuits. While signal echoes may be disturbing to humans they do not affect modem encoded data and may even be used for signal verification purposes. However, the echo suppression circuits act to block digital transmissions. Thus disabling features have been added to these echo suppression or canceller systems, and are activated upon initial transmission of a particular and continuous audio tone (i.e., 2100 H.sub.z) for a given interval by the modem prior to data transmission. The modem approach and analog networks are however not only low speed but fundamentally inconsistent with the digital switches and PCM channel banks which are the current state of the art. They offer no answer to the need for employment of the full capabilities of a data connection system of the "Datapath" type with voice transmissions. In order to obtain the versatility inherent in the "T Link" protocol it is necessary to assure that voice messages are duplexed without echo but that there is no blockage of data messages, and further that the needed interchanges can take place to assure compatibility of data rates.